The data from the borehole scanners (e.g. azimuth, dip width of fractures) was placed into a database. Evaluation of spherical fracture parameters was carried out on Schmidt's stereonets.

The figure below represents the evaluation of all open fractures (> 1mm) of the BK area after weighting and correction of geological bias (2705 individual measurements). This shows that the fractures are orientated in a NW-SE direction with dips of around 80 to 90 degrees. The slight lack of symmetry represent small undulations in the strike of the fracture.

Polar Density Plots of OPen Fractures - EFP

Evaluation of scalar fracture data

Scalar fracture parameters are the length, spacing and aperture of the fractures. The tunnel floor at the BK area (some 324 m2) was mapped with regard to dip, strike and length of discontinuities. The lengths of the fractures were collected, recorded and plotted on a graph. This shows that the majority of the fractures have a length between 1 and 4 meters.

Evaluation of Scalar Fracture Data - EFP

Experimental variogram

Geostatistical approaches such as variograms can be used to determine the frequency and the arrangement of fractures in a rock. This can lead to a better understanding of the hydraulic properties of fractured rock.

Experimental Variogram - EFP
Experimental semi-variogram of the fracture density, data from borehole EFP 98.020

The variogram of fracture spacing is calculated by numbering the fractures observed in the drill core and the fracture distances are assigned to these numbers.

In general, many of the boreholes show a spatial correlation in the fracture density. The sill is reached between 15 m and 20 m. This scale, however corresponds with independent investigations at boreholes and results of geo-electrical measurement. Hence, the BK area lies more or less aside a fault and fracture which characterised by a higher fracture density than other parts of the Grimsel Test Site. It seems therefore at this stage of investigations very likely that solute transport process does not follow single fractures but occurs in a fracture network.


Effective Field Parameters (EFP)- Geological & Geophysical Investigations